Precision liquid dispensing systems have been successfully employed in many applications where safe and accurate handling of fluids is required. Precision liquid delivery is an important function in the production and research of many products, especially for the medical and pharmaceutical industries. Ease of use and reliability, combined with accuracy, are extremely important in the successful performance of each application.
There are several main technologies which are commonly used in dispensing fluids in the micro-liter range. These include piston pumps, peristaltic pumps, time pressure systems, diaphragm pumps and gear pumps. Each of these technologies must incorporate some sort of valving technology along with a method for displacing fluid. It is generally understood that, in order to achieve accurate and reliable dispenses, positive displacement mechanisms have exhibited the best performance. Most positive displacement mechanisms incorporate out of phase check valves and a piston or other positive displacement mechanism to create suction and discharge.
Rotary reciprocating pumps are also commonly used in the industry in order to accomplish small volume dispensing. Rotary reciprocating pumps utilize synchronous rotation and reciprocation of a piston in a precisely mated cylinder bore. One pressure and one suction stroke are completed per cycle. A duct on the piston connects a pair of cylinder ports alternatively with the pumping chamber, i.e., one port on the pressure portion of the pumping cycle and the other on the suction cycle. A pump head module containing the piston and cylinder is mounted in a manner that permits it to be swiveled angularly with respect to the rotation drive member. The degree of angle controls stroke length and in turn flow rate. The direction of the angle controls flow direction. This type of pump has been found to perform accurate transfers of both gaseous and liquid fluids. A typical rotary reciprocating pump is shown in U.S. Pat. No. 4,008,003 to Pinkerton.
A variation of the conventional rotary reciprocating pump is disclosed in U.S. Pat. No. 5,312,233 to Tanny et al. Tanny et al. disclose a rotary/reciprocating liquid dispensing pump having a piston selectively positioned in alignment with one of a plurality of circumferentially spaced small diameter radial passages of a pump housing. Rotation of the piston is achieved conventionally by a stepping motor. However, the rotary motion of the piston is achieved by an electromagnetic clutch/braking mechanism concentrically surrounding a lead screw. The stepping motor is incrementally pulsed and the electromagnetic clutch/braking mechanism energized for selective incremental rotation of the lead screw.
The metering pumps of the prior art suffer from several disadvantages. One disadvantage of the prior art metering pumps is that the rotational and the linear motion of the piston are tied together. In other words, it is not possible with the prior art pumps to independently control the speed of rotational and linear motion of the piston. This has the effect of limiting the accuracy of dispensed liquid volume that can be achieved. In some applications, where it is necessary to provide extremely precise flow rates from inflow and/or outflow ports, it is possible to carefully adjust the angular orientation of a rotary reciprocating pump head module to achieve the desired accuracy. However, this is a difficult hit-or-miss, trial by error procedure that is very time consuming.
Another problem with some prior art dispensing systems is that, once the desired volume of liquid has been dispensed, there is typically some remnant liquid left on the nozzle tip that can sometimes inadvertently drip. In order to prevent excess liquid from dripping from the nozzle, some prior art systems utilize a secondary valve, such as a solenoid valve or a diaphragm valve, to draw back any remnant fluid back into the nozzle. However, a problem with these secondary valves is the added cost and complexity to the system. Additionally, a secondary valve can further add to maintenance, leakage and air bubble problems. Air bubbles in the system obviously decreases accuracy in the dispensed liquid volume.
Additionally, in some applications where, for example, a suspension is to be pumped, it is often desirable to continuously agitate the suspension. This is conventionally accomplished through shaking or stirring means. Again, providing a system with a means to shake or stir liquid during idle dispensing periods not only adds cost and complexity to the system, but can also have detrimental effects such as the introduction of air bubbles to the system. Furthermore, such conventional systems only provide mixing in the external reservoirs and do not provide for mixing within the fluid path.
Finally, conventional system designs are typically large because they do not include multiple pistons in a single housing. As a result, longer and more complex tubing runs are typically provided. When delivering small volumes of liquid, it is desirable to minimize the total fluid path and priming volumes.
Accordingly, it would be desirable to provide a system that addresses these drawbacks of the prior art metering pumps. In particular, it would be desirable to separate control of the rotational and linear motions of the pump piston so that any sequencing of piston rotation and linear movement can be achieved. By separating control of rotary and linear piston motions, increased dosage volume accuracies can be accomplished and any variety of operating methods appropriate for different fluid properties can be implemented.